Floating gate devices: operations and compact modeling

The goal of this book is twofold. First, it explains the principles and physical mechanisms of Floating Gate device operations. Second, starting from a general overview on Compact Modeling issues, it illustrates features and details of a complete Compact Model of a Floating Gate device, the building block of Flash Memories, one of the “hottest” products in the semiconductor industry. Flash Memories are one of the most innovative and complex types of high-tech, nonvolatile memories in use today [see, for example, Proceedings of the IEEE, Special Issue on: Flash Memory Technology, April 2003]. Since their introduction in the early 1990s, these products have experienced a continuous evolution from the simple first products to emulate EPROM memories, to the extreme flexibility of design application in today products. This is an enabling technology: future limits are beyond our current expectations and limited only by our imagination.In the memory arena, Flash memory is the demonstration of the pervasive use of new electronic applications in our lives. Every new application can exploit this flexible and powerful memory technology, either as a stand-alone component or integrated as the enabling feature of the whole silicon integration.Flash are not just memories, they are “complex systems on silicon”: they are challenging to design, because a wide range of knowledge in electronics is required (both digital and analog), and they are difficult to manufacture. Physics, chemistry, and other fields must be integrated; and conditions must be carefully monitored and controlled in the manufacturing process.Memories demand massive investments in R&D, but they also reward with enormous potential market values. Flash memory market (considered the most important market segment among nonvolatile memories) is expected to progress at a very fast pace, and to gain the second place in the overall memory market. This is due to the optimization of cost/performance tradeoffs, and in particular to the inherent flexibility and versatility of this memory, which brings benefits in many applications.The leading application is in multimedia systems, which require memories that are increasingly larger in size, and demand ever-increasing performance characteristics. Telecommunications, computers, automotive and consumer electronics are some additional areas where these memories make possible numerous emerging applications.Moreover, the Flash memory integration is one of the irreplaceable requirements for further technological innovations, and particularly to realize the so-called system on silicon.Compact Model (CM) means an analytic model of the electrical behavior of a circuit element. Modeling is usually aimed at providing means to simulate the behavior of a device or a circuit by quantitative calculation. CM allows to highlight basic properties of a device, thus making easier the understanding and the synthesis of robust circuits. Therefore, the main intent of modeling is to forecast the behavior of a system. This holds for all integrated devices (resistors, capacitors, inductors, transistors, and also the device subject of this book: the floating gate device) and circuits. Compact Models of Floating Gate devices have the same purpose of all compact models: to be used within a program for circuit simulation. The Floating Gate transistor is the building block of a full array of memory cells and a memory chip. In a first approximation, the reading operation of a FG device, and for some cases also programming and erasing, can be considered a single-cell operation. Nevertheless, CMs are fundamental to simulate the effects of the cells not directly involved in the operation under investigation and the effects of the parasitic elements. Furthermore, they allow the simulation of the interaction with the rest of the device, and hence they are useful to check the design of the circuitry around the memory array: algorithms for cell addressing, charge pump sizing taking into account current consumption and voltage drops, etc…